1. Field of the Invention
The present invention is broadly concerned with fill compositions and methods useful for protecting the surfaces forming the contact and via holes during dual damascene processes for the production of integrated circuits. More particularly, the compositions of the invention comprise a quantity of solid cross-linkable components including a polymer binder, and a solvent system for the solid components. The boiling point of the solvent system is preferably sufficiently lower than the cross-linking temperature of the composition so that essentially all of the solvent system is evaporated during the first stage bake without the fill composition being cross-linked to any appreciable degree. In use, the fill compositions are applied to a substrate previously patterned with contact or via hole according to conventional methods followed by heating the composition to its reflow temperature in order to evaporate the solvent system and cause the composition to flow into the hole for uniform coverage. The composition is then cured and the remainder of the dual damascene process carried out in the usual fashion.
2. Description of the Prior Art
The damascene process, or the process of forming inlaid metal patterning in preformed grooves, is generally a preferred method of fabricating interconnections for integrated circuits. In its simplest form, the dual damascene process starts with an insulating layer which is first formed on a substrate and then planarized. Horizontal trenches and vertical holes (i.e., the contact and via holes) are then etched into the insulating layer corresponding to the required metal line pattern and hole locations, respectively, that will descend down through the insulating layer to the device regions (if through the first insulating layer, i.e., a contact hole) or to the next metal layer down (if through an upper insulating layer in the substrate structure, i.e., a via hole). Metal is next deposited over the substrate thereby filling the trenches and the holes, and thus forming the metal lines and the interconnect holes simultaneously. As a final step, the resulting surface is planarized using the known chemical-mechanical polish (CMP) technique, and readied to accept another dual damascene structure.
During the dual damascene process, the contact and via holes are typically etched to completion prior to the trench etching. Thus, the step of trench etching exposes the bottom and sidewalls (which are formed of the insulating or dielectric layer) of the contact or via holes to over-etch which can deteriorate the contact with the base layer. An organic material is therefore used to partially or completely fill the via or contact holes and to protect the bottom and sidewalls from further etch attack. These organic fill materials can also serve as a bottom anti-reflective coating (BARC) to reduce or eliminate pattern degradation and linewidth variation in the patterning of the trench layer, provided the fill material covers the surface of the dielectric layer.
Fill materials have been used for the past several years which have high optical density at the typical exposure wavelengths. However, these prior art materials have limited fill properties. For example, when the prior art compositions are applied to the via or contact holes formed within the substrate and to the substrate surface, the films formed by the compositions tend to be quite thin on the substrate surface immediately adjacent the holes, thus leading to undesirable light reflection during subsequent exposure steps. Also, because the prior art compositions etch more slowly than the dielectric layer, the unetched fill compositions provide a wall on which the etch polymer will deposit. This etch polymer build-up then creates undesirable resistance within the metal interconnects of the final circuit. These problems are explained in more detail below.
There is a need in the art for contact or via hole fill materials which provide complete coverage at the top of via and contact holes. Furthermore, this material should provide adequate protection to the base of the via and contact holes during etching to prevent degradation of the barrier layer and damage to the underlying metal conductors. In order to prevent sidewall polymer buildup, the etch rate of the material should be equal to or greater than the etch rate of the dielectric material, or the contact or via holes should be filled partially so that the fill material in the holes does not extend above the base of the trench following trench etch.
The instant invention overcomes the problems in the art by providing a fill material or composition which can be applied to via and/or contact holes during damascene processing to provide complete surface coverage while avoiding undue buildup of the etch polymer around the top edge of the holes at the base of the trench of the damascene structure.
In more detail, the compositions (fill material and fill composition are used interchangeably herein) of the invention comprise a quantity of solid components including a polymer binder or resin, and a solvent system (either single or multiple solvents) for the solid components. The inventive compositions are superior to prior art compositions in that they are formulated to achieve two requirements: the inventive composition will freely and evenly flow into the contact or via holes with minimal or no cross-linking of the composition during the pre-bake stage (i.e., first stage bake); and during the pre-bake stage essentially all of the solvent is evaporated so that the composition incurs very little shrinkage during the final bake stage. These two requirements are quantified by subjecting the composition to the xe2x80x9cpre-bake thermal stability testxe2x80x9d and the xe2x80x9cfilm shrinkage testxe2x80x9d set forth in detail below.
There are numerous factors which affect the ability of the fill composition to meet the foregoing requirements. For example, the polymer binder or resin preferably comprises an aliphatic backbone and has a molecular weight of less than about 80,000, preferably less than about 25,000, and more preferably from about 2000-7500. Suitable polymer binders include polyesters, polyacrylates, polyheterocyclics, polyetherketones, polyhydroxystyrene, polycarbonates, polyepichlorohydrin, polyvinyl alcohol, oligomeric resins (such as crown ethers, cyclodextrins, epoxy resins), and mixtures of the foregoing. The solvent systems utilized in the composition of the invention preferably have a boiling point of less than about 160xc2x0 C., more preferably less than about 140xc2x0 C., and most preferably less than about 120xc2x0 C. The solvent system should also have a flash point of greater than about 85xc2x0 C., and more preferably greater than about 100xc2x0 C. When more than one solvent is utilized in the solvent system, the boiling point or flash point of the solvent system refers to the boiling point or flash point of the highest boiling or lowest flashing solvent. It is also important that the solvent system be compatible with the resist system chosen for the particular damascene process. That is to say, an air-dried film of the fill composition should redissolve in the chosen resist solvent system within 30 seconds with essentially no undissolved residue being visible in the solution.
The concentrations of the solvent system and other volatile species present in the composition is not critical, so long as the total concentration of the solvent system and volatile species in the film just prior to cross-linking of the film (i.e., just prior to the second stage bake) is less than about 5% by weight, and preferably less than about 2% by weight, based upon the total weight of the fill composition taken as 100% by weight. This solvent system and volatile weight percent in combination with the above solvent system boiling and flash points is important to ensure that minimal shrinking of the composition occurs during the second stage bake. Preferred solvents for use in the solvent system include alcohols, ethers, glycol ethers, amides, esters, ketones, water, propylene glycol monomethyl ether (PGME), propylene glycol monomethyl ether acetate (PGMEA), ethyl lactate, and PCBTF (p-chlorobenzotrifluoride), with PGME being particularly preferred.
The fill compositions of the inventions preferably cross-link at a temperature of from about 150-220xc2x0 C., and more preferably about 180xc2x0 C. It is important that the fill compositions cross-link at a temperature higher than the temperature to which the composition is heated during the first stage reflow baking so as to avoid undue cross-linking of the composition during the reflow step. Such premature cross-linking would prevent the composition from completely and uniformly flowing into the contact or via holes. Cross-linking of the polymer binder in the composition can be accomplished by the use of a cross-linking agent in the composition or by the selection of polymer binders which include xe2x80x9cbuilt inxe2x80x9d cross-linking moieties. Preferred cross-linking systems include acid or base catalyzed, thermal catalyzed, and photocatalyzed systems such as aminoplasts, epoxides, blocked isocyanates, acrylics, and mixtures thereof.
All solid components utilized in the fill compositions of the invention should form a free-flowing liquid at a first stage reflow bake temperature of less than about 200xc2x0 C., and preferably less than about 120xc2x0 C., thus preventing the composition from adhering to the hole sidewalls and forming a steep meniscus. All components must remain chemically stable at these reflow temperatures for at least about 15 seconds, and preferably at least about 30 seconds. By chemically stable, it is meant that the components only undergo changes in their physical state and not in their chemical state (such as by cross-linking of their components). The chemical stability can be determined by UV/VIS or FTIR analysis, both before and after the first stage bake.
In order to avoid the etch polymer buildup problems of the prior art, the etch rate of the fill composition should be approximately equal to the base material or dielectric material etch rate. Furthermore, the fill composition should have a faster etch rate than the etch rate of the photoresist. The ratio of the composition etch rate to the photoresist etch rate should be at least about 1.5:1, preferably at least about 3:1, and more preferably at least about 4:1. One way to achieve such fill composition etch rates is through the selection of the polymer binder. Highly oxygenated or halogenated species will result in an increased etch rate.
The compositions can also be formulated to include optional ingredients as necessary. Optional ingredients include wetting agents (such as fluorinated surfactants, ionic surfactants, non-ionic surfactants, and surface active polymer additives) and dyes or chromophores. Examples of suitable dyes include any compound that absorbs at the electromagnetic wavelength used for the particular process. Examples of dyes which can be used include compounds containing anthracene, naphthalene, benzene, chalcone, phthalimides, pamoic acid, acridine, azo compounds, and dibenzofuran. The dyes may be physically mixed into the composition, or alternately, may be chemically bonded to the polymer binder. For e-beam exposure, conductive compounds can be used.
The method of applying the fill compositions to a substrate with a contact or via hole simply comprises applying a quantity of a composition hereof to the substrate surfaces forming the hole by any conventional application method (including spin coating). After the composition is applied to the hole, it should be heated to its reflow temperature (as set forth above) during the first stage bake so as to cause the composition to flow into the contact or via hole(s), thus achieving the desired hole and substrate surface coverage. After the desired coverage is achieved, the resulting fill composition film should then be heated to at least the cross-linking temperature of the composition so as to cure the film.
In partial fill processes, the height of the cured fill material in the hole should be from about 35-65%, and preferably at least about 50% of the depth of the hole. In complete fill processes, the height of the cured fill material in the hole should be at least about 95%, and preferably at least about 100% of the depth of the hole. The height of the meniscus of the cured fill composition should be less than about 15% of the depth of the hole, and preferably less than about 10% of the hole depth. Although a meniscus is conventionally deemed to be a concave surface or xe2x80x9cvalleyxe2x80x9d which forms on the top surface of a flowable substance in a container (i.e., the via or contact hole), as used herein the term meniscus is also intended to include convex surfaces or xe2x80x9chillsxe2x80x9d formed on the top surface of a substance in a container or hole. The meniscus height as used herein refers to the distance from the highest point at which the composition contacts the sidewalls of the contact or via holes to the lowest point in the concave surface of the meniscus, or for a convex meniscus, the distance from the highest point at which the composition contacts the sidewalls of the contact or via holes to the highest point on the convex surface.
The thickness of the cured fill material film on the surface of the substrate adjacent the edge of the contact or via hole should be at least about 40%, preferably at least about 50%, and more preferably at least about 70% of the thickness of the film on the substrate surface a distance away from the edge of the contact or via hole approximately equal to the diameter of the hole. Finally, the percent of solids in the compositions should be formulated so that the thickness of the film formed on the substrate surface is from about 35-250 nm. Following the methods of the invention will yield precursor structures for the dual damascene process having the foregoing desirable properties.